Lasers are widely used in a variety of commercial and research applications. There are several types of lasers in use including, for example, gas lasers, solid-state lasers, free electron lasers, and semiconductor lasers. Semiconductor lasers are generally fabricated on polished semiconductor wafers with lithographic techniques and include an active region disposed in a cavity formed between top and bottom mirrors. In operation, the active region may be pumped (e.g., electrically or optically) with a pumping energy to produce photons, some of which resonate and build up to form coherent light in the cavity. A portion of the coherent light resonating in the cavity may pass through one of the mirrors (exit mirror) as a laser beam. The exit mirror generally has slightly lower reflectance (i.e., reflectivity) than the other mirror.
Semiconductor lasers may be edge-emitting semiconductor lasers or surface-emitting semiconductor lasers. Edge-emitting semiconductor lasers may output the laser beam in a direction parallel to the wafer surface, whereas the surface-emitting semiconductor lasers output the laser beam in a direction perpendicular to the wafer surface. Generally, surface-emitting layers may emit light more efficiently than edge-emitting lasers and may also be arranged in two-dimensional, light-emitting arrays.
One type of surface-emitting semiconductor laser is the vertical-cavity surface-emitting laser (VCSEL). In a VCSEL, the term “vertical” is the direction perpendicular to the plane of the substrate on which the constituent layers are deposited or epitaxially grown, with “up” being typically defined as the direction of epitaxial growth. VCSELs may be configured as top-emitting or bottom-emitting. In a top-emitting configuration, the output laser beam is emitted out of the top side of the VCSEL, in which case the top mirror is the exit mirror. Conversely, in a bottom-emitting VCSEL, the laser beam is emitted from the bottom side of the VCSEL, in which case the bottom mirror is the exit mirror.